The present invention relates to alarms in medical diagnostics apparatus, and in particular to improvements in reducing nuisance alarms for pulse oximeters.
A typical pulse oximeter measures two physiological parameters, percent oxygen saturation of arterial blood hemoglobin (SpO2) and pulse rate. For alarm purposes, low and high thresholds are set for both SpO2 and pulse rate, defining normal ranges within which it is desired to maintain the patient. For example, with a neonate it might be desired that sat should remain between 85 and 95 percent and pulse rate should remain between 120 and 170 beats per minute. From the two measured parameters, typically four alarm types can be generated, low sat, high sat, low rate, and high rate. In some pulse oximeters, an alarm begins immediately when either sat or rate goes outside the normal range and the alarm ends immediately when both sat and rate return within the normal range. Alarms are typically announced by audible and/or visual indicators. Alarms, which are dependent on the instantaneous excursions of a measured value outside a range, are commonly referred to as conventional alarms.
Each occurrence in which a measured parameter goes outside the normal range is referred to as an event. Thus, in a typical pulse oximeter, each event coincides with an alarm, and the alarm duration may be identical to the event duration. Some of the alarms produced by typical pulse oximeters are not generally considered to correspond to events that are clinically significant. The exact definition of clinical significance varies depending on the patient and circumstances, but is in general related to the severity and duration of the event of interest. For example, a very shallow desaturation might only be considered significant if sustained for a relatively long period of time. Likewise, a desaturation of very brief duration might only be considered significant if it falls very deep below the low sat threshold. In addition to clinically insignificant alarms, parameter measurement error due to noise, signal artifact or bias can also produce false events and trigger alarms. An alarm that does not correspond to a clinically significant event may be considered a nuisance alarm.
Several approaches are available which attempt to reduce the number of nuisance alarms. Some of these approaches have either looked at lowering the alarm threshold or waiting some fixed period of time after the threshold has been crossed before triggering an alarm. Lowering the threshold can be problematic because a patient's blood oxygen saturation can remain indefinitely below the original threshold, but above the new threshold, and an alarm will never be generated. Delaying alarm generation by a fixed amount of time is also problematic due to a potentially serious situation in which a patient's saturation abruptly falls to and remains at a very low level, requiring prompt medical attention.
Another solution to the nuisance alarm problem is described in U.S. Pat. No. 5,865,736, entitled, “METHOD AND APPARATUS FOR NUISANCE ALARM REDUCTIONS,” assigned to the assignee herein. The solution described by the '736 patent is commercially known as the SatSeconds™ Alarm Management Technology (“SatSecond”) feature. The SatSecond concept has been incorporated into some of assignee's pulse oximeters, such as the model N-395 pulse oximeter, for enhanced alarm management. FIG. 1 is a graph illustrating the alarm response according to this known SatSecond approach. This figure shows a conventional and the SatSeconds alarm management methods. This figure, for illustration purposes shows the methods applied to SpO2 measurements. As described above and shown in FIG. 1, with conventional alarms, SpO2 (4) or pulse rate (not shown) readings that fall below a specified fixed lower threshold 6 or above a specified fixed upper threshold (not shown) trigger an audible or visible alarm state. With the SatSecond methodology, an alarm state is entered only when the second-by-second accumulated product 2, of time and the degree to which the SpO2 (4) exceeds the lower 6 or upper (not shown) specified threshold, equals or exceeds an integrated threshold 8. Both the conventional and SatSecond alarm management methods are equally applicable to pulse rate or other physiological measurements.
The motivation for the SatSecond method is to reduce the number of nuisance alarms in which a measured value such as SpO2 is beyond an alarm threshold, but does not represent a clinically significant event. For example, if a caregiver feels that a desaturation of less than 5 points below the lower alarm threshold for less than 5 seconds is not clinically meaningful, but rather constitutes a nuisance, the caregiver may set the SatSecond alarm threshold to “25” (5 points for 5 seconds). Then only a deeper desaturation of longer duration (i.e., a product that exceeds 25 SatSeconds) will initiate an alarm. In certain pulse oximeter models manufactured by the assignee herein, the product of saturation-below-the-threshold and time are accumulated once per second, and this product is compared to the SatSecond alarm threshold each time is it calculated. The effect of using the SatSecond alarm management method is to reduce the number of nuisance alarms and to alarm more specifically in response to events that are clinically meaningful as established previously by the caregiver.
A limitation in the use of the each of these prior art methods occurs when the SpO2 value (or other measured value) is systematically in error, as in where there is a high or low bias in the measured value, even if the bias error is relatively small. Using the SatSecond method as an example, this limitation is illustrated in FIG. 2. The graph 22 shows a monitored value of SpO2 having a bias of a few points high relative to the true saturation 21. As the desaturation event 25 occurs, the lower alarm threshold 24 is not reached until later in the event, if at all, and the SpO2 value dips only slightly below the threshold 24. Accordingly, the SatSecond value 28 (which corresponds to the area of the dark hatched region 26 of the upper curve 22 below the lower alarm threshold 24) never achieves the necessary level 29 needed to initiate an alarm state. FIG. 2 provides an illustration of a “missed” SatSecond alarm due to a bias in the SpO2 readings. The erroneously high SpO2 value may interfere with the ability to accurately calculate the proper value of the SatSecond integral 28. The converse (i.e., false SatSecond alarm) would occur if the SpO2 readings were too low due to a low bias. Hence, SpO2 bias affects the reliability of measured values and alarms based on those values.
Ideally, the SpO2 reading will be proper (i.e., unbiased from the true SaO2). However, under some circumstances such a bias can and does occur. It is known that bias can be created, for example, by an improperly placed sensor that shunts light between the emitter and the detector, or by a sensor that has been applied too tightly, or a by patient with significant edema. Additionally, sensor placement variations, as well as other factors introduce bias, such that even instrument specifications acknowledge the presence of bias. Specifically, the accuracy specification for pulse oximetry sensors readily allows a bias between two sensors placed on the same patient of 3 sat-points. Under such circumstances (i.e., two sensors placed on the same patient), one sensor may indicate an alarm state, while the other does not, resulting in ambiguity in not knowing which sensor is providing the more correct reading. Thus, although the SatSecond invention greatly reduces nuisance alarms in pulse oximeter readings, the measurements and hence alarm events may still be susceptible to bias-induced nuisance alarms. Moreover, the SatSecond improvement is based on a product of saturation-below-a-fixed threshold (or above) and time. This fixed threshold can also be problematic, as is described below.
Alarm thresholds described thus far are based on fixed windows, where a window is defined by the region between a fixed lower and a fixed upper alarm threshold. The fixed lower and upper threshold values are based on typical default values used for patients in general, and which may be set by the caregiver irrespective of the current instrument readings. However, the fixed window approach may be problematic for patients having, for example, a chronically elevated pulse rate value. Some prior art pulse oximeters manufactured by the assignee herein offered a feature known as “Smart Alarms” to allow caregivers to quickly establish the lower and upper conventional alarm thresholds by manually pressing a button on the oximeter unit. The “Smart Alarm” is essentially a fixed relative threshold based on a current physiological value that is being monitored. Using this “Smart Alarm” feature, the conventional alarm thresholds could be established at a preset value above and below the current readings of pulse rate, as opposed to the fixed default values typically used for patients in general. Thus if a patient is chronically at an elevated pulse rate, a revised fixed threshold relative to the current readings could be easily set to a preset number below the current reading so as not to alarm unnecessarily. While the “Smart Alarm” approach allows for the setting of a new fixed threshold that is related to the then current readings, it is still a fixed threshold and hence suffers from the same shortcomings described thus far.
There is therefore a need for improvements in medical diagnostic devices, and in particular to improvements in both integrated or “product”-type and relative-deviation threshold alarms for pulse oximeters.